I believe auction markets of different servers and different levels are different. To minimize tokens needed for bidding, you should observe, understand it, and then determine your strategy. There is no universal optimum number of tokens for each bid.

In this season, I have 2 different level teams in the same server. The auction market for high level team is far tougher than that for low level team. As I checked earlier, in average I spent approximately 6 and 1.5 tokens to acquire one new player for the high level team & low level team respectively (including those tokens wasted in unsuccessful biddings). As I only got 35 tokens per season, my general strategy is spending maximum 3 tokens on each auction.

Providing that cash is sufficient, we certainly prefer to acquire young players. But usually there are more competitions to acquire them. For my higher level team, I often have to focus to buy slightly older players because I lack tokens to compete for the most attractive young players.